what is t3

T3, or triiodothyronine, is a hormone produced by the thyroid gland. It works together with another hormone called thyroxine (T4) to regulate how the body uses energy, control weight, body temperature, muscle strength, and the nervous system. T3 comes in two forms: bound T3, which attaches to protein, and free T3, which does not attach to anything. A T3 test is a blood test that measures the level of T3 in the blood and is used to diagnose thyroid conditions, particularly hyperthyroidism (overactive thyroid) . If T3 levels are not normal, it can be a sign of thyroid disease.